It’s a lot of work to design, build, test and maintain a web application. Many
Python and Django projects share common problems. Wouldn’t it be great if we
could save some of this repeated work?

Reusability is the way of life in Python. The Python Package Index (PyPI) has a vast range of packages you can use in your own
Python programs. Check out Django Packages for
existing reusable apps you could incorporate in your project. Django itself is
also just a Python package. This means that you can take existing Python
packages or Django apps and compose them into your own web project. You only
need to write the parts that make your project unique.

Let’s say you were starting a new project that needed a polls app like the one
we’ve been working on. How do you make this app reusable? Luckily, you’re well
on the way already. In Tutorial 1, we saw how we
could decouple polls from the project-level URLconf using an include.
In this tutorial, we’ll take further steps to make the app easy to use in new
projects and ready to publish for others to install and use.

Package? App?

A Python package provides a way of grouping related Python code for
easy reuse. A package contains one or more files of Python code (also known
as “modules”).

A package can be imported with importfoo.bar or fromfooimportbar. For a directory (like polls) to form a package, it must contain
a special file __init__.py, even if this file is empty.

A Django application is just a Python package that is specifically
intended for use in a Django project. An application may use common Django
conventions, such as having models, tests, urls, and views
submodules.

Later on we use the term packaging to describe the process of making a
Python package easy for others to install. It can be a little confusing, we
know.

You created mysite/templates in Tutorial 7,
and polls/templates in Tutorial 3. Now perhaps
it is clearer why we chose to have separate template directories for the
project and application: everything that is part of the polls application is in
polls. It makes the application self-contained and easier to drop into a
new project.

The polls directory could now be copied into a new Django project and
immediately reused. It’s not quite ready to be published though. For that, we
need to package the app to make it easy for others to install.

The current state of Python packaging is a bit muddled with various tools. For
this tutorial, we’re going to use setuptools to build our package. It’s the
recommended packaging tool (merged with the distribute fork). We’ll also be
using pip to install and uninstall it. You should install these
two packages now. If you need help, you can refer to how to install
Django with pip. You can install setuptools
the same way.

Python packaging refers to preparing your app in a specific format that can
be easily installed and used. Django itself is packaged very much like
this. For a small app like polls, this process isn’t too difficult.

When choosing a name for your package, check resources like PyPI to avoid
naming conflicts with existing packages. It’s often useful to prepend
django- to your module name when creating a package to distribute.
This helps others looking for Django apps identify your app as Django
specific.

Application labels (that is, the final part of the dotted path to
application packages) must be unique in INSTALLED_APPS.
Avoid using the same label as any of the Django contrib packages, for example auth, admin, or
messages.

=====Polls=====
Polls is a simple Django app to conduct Web-based polls. For each
question, visitors can choose between a fixed number of answers.
Detailed documentation is in the "docs" directory.
Quick start-----------1. Add "polls" to your INSTALLED_APPS setting like this::
INSTALLED_APPS = [
... 'polls', ]2. Include the polls URLconf in your project urls.py like this::
path('polls/', include('polls.urls')),
3. Run `python manage.py migrate` to create the polls models.
4. Start the development server and visit http://127.0.0.1:8000/admin/
to create a poll (you'll need the Admin app enabled).
5. Visit http://127.0.0.1:8000/polls/ to participate in the poll.

Create a django-polls/LICENSE file. Choosing a license is beyond the
scope of this tutorial, but suffice it to say that code released publicly
without a license is useless. Django and many Django-compatible apps are
distributed under the BSD license; however, you’re free to pick your own
license. Just be aware that your licensing choice will affect who is able
to use your code.

Next we’ll create setup.cfg and setup.py files which detail how to
build and install the app. A full explanation of these files is beyond the
scope of this tutorial, but the setuptools documentation has a good explanation.
Create the files django-polls/setup.cfg and django-polls/setup.py
with the following contents:

Only Python modules and packages are included in the package by default. To
include additional files, we’ll need to create a MANIFEST.in file. The
setuptools docs referred to in the previous step discuss this file in more
details. To include the templates, the README.rst and our LICENSE
file, create a file django-polls/MANIFEST.in with the following
contents:

It’s optional, but recommended, to include detailed documentation with your
app. Create an empty directory django-polls/docs for future
documentation. Add an additional line to django-polls/MANIFEST.in:

recursive-includedocs*

Note that the docs directory won’t be included in your package unless
you add some files to it. Many Django apps also provide their documentation
online through sites like readthedocs.org.

Try building your package with pythonsetup.pysdist (run from inside
django-polls). This creates a directory called dist and builds your
new package, django-polls-0.1.tar.gz.

Since we moved the polls directory out of the project, it’s no longer
working. We’ll now fix this by installing our new django-polls package.

Installing as a user library

The following steps install django-polls as a user library. Per-user
installs have a lot of advantages over installing the package system-wide,
such as being usable on systems where you don’t have administrator access
as well as preventing the package from affecting system services and other
users of the machine.

Note that per-user installations can still affect the behavior of system
tools that run as that user, so virtualenv is a more robust solution
(see below).

Earlier, we installed the polls app as a user library. This has some
disadvantages:

Modifying the user libraries can affect other Python software on your system.

You won’t be able to run multiple versions of this package (or others with
the same name).

Typically, these situations only arise once you’re maintaining several Django
projects. When they do, the best solution is to use virtualenv. This tool allows you to maintain multiple
isolated Python environments, each with its own copy of the libraries and
package namespace.